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EEG Alpha Oscillations The inhibition-timing hypothesis. Brain neuroimaging methods. Direct electrode recording Recorded by electrodes penetrating the cortex Best spatial & temporal resolution Highly invasive Electrocorticography ( ECoG ) Recorded from the surface of Cortex
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Brain neuroimaging methods • Direct electrode recording • Recorded by electrodes penetrating the cortex • Best spatial & temporal resolution • Highly invasive • Electrocorticography (ECoG) • Recorded from the surface of Cortex • Less invasive than direct recording • High temporal resolution • Better spatial resolution than EEG • fMRI • Measures hemodynamic response in the brain • Millimeter spatial resolution • Poor temporal resolution • High imaging costs
Near-infrared spectroscopy (NIRS) • Measures blood oxygen variation • High temporal and spatial resolution • Magnetoencephalography (MEG) • Measures magnetic field produced by brain electrical currents • Non-invasive • Better spatial resolution than EEG • Electroencephalography (EEG) • Measures brain electrical activity from scalp surface • High temporal resolution • Low cost, availability • Non-invasive
Event-Related Potential • Obtained by averaging many trials, time-locked to the stimuli • Good temporal resolution
Alpha Oscillations • Brain wave first observed by Berger • Has the strongest power in awake EEG • Suppressed by eye opening • Conventional theory • Mostly observed during idling • Solely observed as Event-Related Desynchronization (ERD)
The inhibition-timing hypothesis • α can respond with increase in amplitude (ERS): • When subject controls the execution of a response • When subject withholds the execution of a learned motor function • In tasks in which the subject should block accessing to the past memory • On sites that probably are under or exert top-down control • In brain areas that are not task relevant • Traveling alpha waves • Oscillatory activity • is induced by inhibitory cells (not a compulsory assumption) • reflects phases of maximal and minimal inhibition • Timing • Rhythmic oscillations provide limited time window for firing for many neurons
Excitatory postsynaptic potential (EPSP): a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell that increases the chance a future action potential will occur in a postsynaptic neuron • Inhibitory postsynaptic potential (IPSP): a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron • Inhibitory Cells: Cells that release GABA (as the main inhibitory neurotransmitter) • Excitatory Cells: Cells that release Glutamate (as the main excitatory neurotransmitter) - Only 15% of cortical cells are GABAergic (inhibitory) - Only 10% of synchronously active neurons are capable of generating an amplitude which is about 10-times of the amplitude of non-synchronized neurons
Inhibition-timing hypothesis • Oscillatory activity reflects the changes between phases of maximal and minimal inhibition • Oscillations provide a small time window for firing of many neurons Tonic firing Rhythmic firing Inhibition
Synchronize alpha activity can be seen In tasks where a learned response must be withhold ERS during Encoding and retention ERD during retrieval Upper alpha peak during retention • ERS reflects inhibitory top-down control during retention Modified Sternberg task
ERS when a learned motor task must be withhold • Experiment by Hummel • Subjects trained for a finger movement task • In ACT condition the subject should look at the cue and perform the movement • In INH condition the subject should look at the cue and withhold performing the movement • ERD during ACT condition • ERS observed during INH condition over the same area • ERS in INH was observed only when subjects were trained for the motor task • Blood Oxygenation Level Dependent (BOLD) signal decreases during INH • ERS exhibits top-down control over accessing the learned memory trace
Tonic alpha power, cognitive performance and inhibition • The extent of ERD also depends on resting alpha power • High resting alpha power and high ERD during a task is associated with good performance in memory task • Perception performance is enhanced if the cortex is already activated (low resting alpha power) • Memory performance is enhanced if the cortex is deactivated before a task (high resting alpha power) • High level of cortical excitation (low resting alpha power) is helpful to analyze a visual input • High level of initial excitation is detrimental when a high selectivity is needed for accessing a memory trace
Intelligence and the inhibition of task-irrelevant brain areas Grabner experiment: Taxi drivers were asked to indicate which place the would pass when driving from A to B • Based on their knowledge of the city roads (Long-Term Memory) • Based on a fictional map they had to learn for the task (Working memory) • A very low ERD was observed for subjects with lower IQ whereas a large and focused ERD for higher IQ • Large and widespread ERD for lower IQ verses a focused ERD and a large widespread ERS for higher IQ More intelligent subjects may only use the brain areas that are directly relevant
Top down and bottom up processing • Top down processing can be seen as processing what one is perceiving using past information. It occurs when someone infers from a generalization, law etc. to conclude something about a particular example, instance, case etc. • Bottom up processing can be seen as starting with no knowledge on a subject. It is said to occur when one draws generalizations from particular examples, instances, cases etc. to capture commonalities between them.
Stroop testERS during inhibition of semantic processing • Read aloud the color of each word below: Your brain alpha power might increase during reading each color in order to block the interference from the semantic processing of each word
Inhibition and top-down control • Top-down process as an attention control function that keeps focused on highly selective aspects of task performance by using inhibition to prevent interference from task-irrelevant brain areas or processing system. • The internal control of attention – particularly under situations when sensory information must not be processed – is associated with an increase in alpha activity(ERS). • with human subjects observing another person doing a complex motor task, ERD over sensory-motor areas but ERS over the Supplementary Motor Area can be observed • in a WM task, Sauseng et al. (2005b) found upper alpha ERS when subjects had to manipulate information during a retention interval.
Sauseng Experiment: • Subjects see a 4x4 matrix with some colored squires • Two tasks were compared: • If the color of squires was red subjects should mirror the image around the vertical line (manipulation) and after retention period compare it with the probe item • If the color of squires was green subjects were required just to remember the original image and after retention period compare it with the probe item • During Manipulation period upper alpha ERS at frontal sites and ERD at posterior sites was larger than during pure retention • Absolute alpha power, showed that during manipulation brain activation reflects a state of “alpha equilibrium”—where alpha power exhibits a topographically flat distribution • This effect was not only found with alpha power but also with alpha frequency. Alpha frequency at frontal sites increased to values similar to those at occipital sites during manipulation. In contrast, during pure retention occipital alpha was clearly faster than prefrontal alpha
Sauseng Test: Top-down processing - It appears that a state of alpha equilibrium enhances or enables cortico-cortical communication. Because during manipulation frontal sites are leading and posterior sites trailing (Fig C), we conclude that anterior sites control in a top-down manner mental operations on the memory trace stored at posterior sites.
Alpha and metabolic rate • large negative correlations between alpha power (at 8–12 Hz, recorded from O1 and O2) and the fMRI– BOLD signal in cortical areas • In some areas, however, positive correlations were also obtained. • It should be noted that the interpretation of correlations between alpha and metabolic rate is problematic because: • Large alpha amplitudes may stem from high synchrony between only a few percent of neurons with the majority of neurons being relatively inactive and exhibiting only a low firing rate thereby producing minimal metabolic activity. • Decreasing synchrony but increasing EPSP amplitudes (leading to increased firing rate) in a larger number of neurons may produce small EEG amplitudes and a large metabolic signal.
Timing hypothesis • Two effects: • In general, decrease in firing rate • Reflects inhibition • Increase in rhythmic discharges • Provides limited time window for firing of neurons • An increase in inhibition (driving an oscillation) is accompanied by an increase in oscillatory activity that results in a stricter timing of neural activity. • oscillations are an extremely useful mechanism to control the time window in which neurons are most likely to fire. • In a complex network, common target cells will tend to receive neural activity synchronously. This increases the likelihood that these cells will be activated selectively and that one brain region might influence another.
Alpha phase controls cortical excitability • Varela’s experiment: • Subjects were asked to judge whether the two (shortly displayed) stimuli are simultaneous or moving • It takes 50ms (due to peripheral transmission time) for a visual stimulus to be processed at the cortex • The stimuli which are displayed during positive alpha cycle perceived as moving. (processed after 50ms when the alpha is in the negative cycle and the cortex is in the more excitatory state, so facilitating stimulus processing) • The stimuli which are displayed during negative alpha cycle perceived as simultaneous (processed after 50ms when the alpha is in the positive cycle and the cortex is in the more inhibitory state, hence delaying stimulus processing)
Alpha phase reset • alpha is capable of controlling the timing of cortical activation in the range of milliseconds • the P1 is the earliest manifestation of a top-down process during early sensory processing • Under conditions where sensory processing is guided by a specific expectancy, e.g., about the spatial location and/or type of stimulus, the P1 amplitude will be larger than under conditions where specific expectancies are lacking. • the P1 is generated (at least in part) by a phase reset of alpha and that the surface positivity of this component represents the deactivating/inhibitory phase of alpha • In the time window of the P1 early semantic classification/identification processes are initiated. • This time window is the earliest where top-down processing can interact with bottom-up processing.
Hanslmayr’s experiment: • Briefly exposed letters “p” and “q” are shown in random followed by a pattern mask • Subject were asked to detect either the letter was ‘q’ or ‘p’ • compared to bad performers, good performers showed a larger P1-N1 complex that was associated with a significantly larger alpha phase locking index (PLI). • good (target categorization/identification) performance is reflected by increased alpha phase locking. • the amplitude of the P1 most likely is generated (at least in part) by a reset of alpha
The lack of desynchronization and the large differences in prestimulus power between good an bad performers is clearly evident in the time/frequency/(wavelet) power plot.
Some Conclutions: • during phase reset cortical excitability is reorganized in a way that those brain areas that are task-relevant are set in a comparatively less excitatory or inhibitory state at exactly that time window when bottom-up sensory processing interacts with top-down processes. • Alpha phase reset may be understood as an early inhibitory filter that enables the emergence of a highly selective and excitatory encoding network.
Schack’s experiment: • investigating alpha phase synchronization during the encoding of spoken words that subjects were instructed to remember. • Four lists containing either 25 concrete or 25 abstract nouns were presented • topographical pattern of alpha phase synchronization showed large differences between concrete and abstract words already in a very early time window of 100–200 ms. • differences in alpha phase synchronization between word categories most likely reflect differences in global categorization processes between abstract and concrete words. • the speed of • traveling alpha waves was faster for abstract than concrete words. Because abstract words are less numerous than concrete words, this may indicate that the search area for abstract words can be narrowed down faster for abstract then concrete words. • event-related alpha coherence emerges at the time window of the P1 and reflects the timing of co-activation between brain sites during interactive top-down and bottom-up processing
Summary • Inhibition • α can respond with increase in amplitude (ERS): • When subject controls the execution of a response • On sites that probably are under or exert top-down control • Oscillatory activity • is induced by inhibitory cells (not a compulsory assumption) • reflects phases of maximal and minimal inhibition • Timing • Rhythmic oscillations provide limited time window for firing for many neurons • Traveling alpha waves, cyclic changes in cortical excitability in the alpha frequency range, alpha coherence, phase synchronization and phase locking are considered manifestations of this timing mechanism. • upper alpha oscillations are related to top-down processes in a complex sensory-semantic LTM system that controls the access to and manipulation with stored information. • during the early period of stimulus encoding, phase-locked alpha reflects the top-down influence of semantic categorization processes.
